The production of integrated systems made of silicon, whether they are power or processing systems, is increasingly carried out with differential structures and structures of variable reference impedance for the analogue portions. The “outside” world for its part remains essentially a system of the common-mode type and with 50Ω reference impedance.
The connection between a balanced transmission line and an unbalanced transmission line cannot be made without an appropriate electric circuit. This transition is provided by a transformer of the balanced-to-unbalanced type called a “balun”.
A balun converts, for example, a signal of the common-mode type into a signal of the differential mode type and vice versa, and performs the impedance transformations. The main electrical characteristics of a balun are its insertion loss, which must be as low as possible, the balancing of the differential channels in phase (180°) and in amplitude (δ=0 dB), and its bandwidth, that is to say the range of frequencies on which the transformer can be used as a “balun” with a balancing of the channels in phase and in amplitude.
Baluns can also be used, for example, in receive and transmit circuits of wireless communication systems, for the design of differential circuits such as amplifiers, mixers, oscillators and antenna systems.
Baluns can be made with transmission lines such as Lange couplers, couplers of the ring type commonly known to those skilled in the art as “Rat-race” couplers, or Marchand couplers, or else couplers with stacked or coplanar inductors.
In the structures with transmission lines, lines of a length equivalent to a quarter wavelength, λ/4, or a half wavelength, λ/2, are used for isolations between channels and to produce the delay of one channel relative to the other. As an example, for frequencies of 2 GHz and 80 GHz, and for a metallic line resting on a material having a constant dielectric ∈r equal to 4, the length values λ/4 and λ/2 correspond to 18.75 mm and 37.5 mm for 2 GHz and 0.468 mm and 0.937 mm for 80 GHz.
Consequently, this transmission-line structure has the drawback of occupying a large area of silicon for microwave applications or of not being integratable for lower-frequency applications. For this reason, the inductor-based structures are preferably used.
Usually, the baluns that are used have one input channel and one output channel; they are called “two-channel baluns”. In some cases, however, several outputs are necessary. This is the case, for example, with a transceiver circuit.
Beginning with a two-channel balun comprising a primary inductive circuit coupled to an antenna for example and a secondary inductive circuit, for producing a transceiver device, it is necessary to couple a Power Amplifier (PA) to the secondary inductive circuit for the transmission of signals and a Low Noise Amplifier (LNA) for the reception of signals and for minimizing the line losses.
However, the coupling of the power amplifier PA and of the low-noise amplifier LNA to the secondary inductive circuit involves coupling the power amplifier PA and the low-noise amplifier LNA together. When the power amplifier PA is operating, its output voltage is high, typically from zero to twice the power supply voltage of the circuit. This high voltage is then directly applied to the gates of the transistors of the low-noise amplifier LNA and induces a considerable risk of destroying these transistors.
To prevent such damage, it is known practice, in a first configuration illustrated in FIG. 1a, to use several baluns B1 and B2. The use of a plurality of baluns B1 and B2 to multiply the number of channels results in a considerable increase in the area of silicon occupied. For example, in the case of using two baluns B1 and B2 in order to have two output channels and different transformation ratios, the total area occupied by the circuit comprising two baluns is doubled.
Moreover, in such a configuration in which two baluns are placed side by side, a capacitive coupling appears between the two baluns and more particularly between the primary inductive circuits I1 and the secondary inductive circuits I2 of the two baluns B1 and B2. This capacitive coupling may cause an imbalance of the channels.
Furthermore, as illustrated in FIG. 1a showing a transceiver circuit using two baluns B1 and B2, the circuit must comprise a set of switches S on the primary inductive circuit I1 in order to make the circuit operate in transmit mode or in receive mode.
However, such a configuration does not make it possible to carry out duplexing, that is to say to have the system operate in transmit and receive mode simultaneously.
Moreover, the switches S introduce an additional loss factor resulting, for the transceiver circuit, in lower efficiency in transmit mode and a lower gain in receive mode. Moreover, it is necessary to use a circuit for controlling the switches S. The circuit for controlling the switches S then introduces a factor of additional risk relating to the reliability of the switches S that may cause the circuit not to function.
In another configuration that is known and is illustrated in FIG. 1b, it is possible to use a single balun, but for which it is necessary to add commutator switches S to the secondary inductive circuit I2 between the power amplifier PA of transmit mode and the low-noise amplifier LNA of receive mode.
This configuration also does not make duplexing possible because of the alternating operation between transmit and receive.
Moreover, the switches S introduce an overall power loss and a reduction in efficiency of the power amplifier PA, and a degradation of the noise factor and a degradation of the overall gain of the low-noise amplifier LNA. Moreover, it is also necessary, in this configuration, to use a circuit for controlling the switches S which introduces an additional risk factor relating to the reliability of the switches S.
Moreover, the transformation ratio of a two-channel balun is fixed. The outputs on the side of the power amplifier PA or on the side of the low-noise amplifier LNA are necessarily differential or single-channel, but it is impossible to have a differential impedance on the side of the low-noise amplifier LNA and single-channel on the side of the power amplifier PA and vice versa.